[1]United States Patent Office 3@421,979 3,421,979 NUCLEAR REACTOR FUEL ELEIMENTS David Lees Linning, Culcheth, Warrington, England, as signor to Uriited Kingdom Atomic Energy Authority, London, England Filed Dec. 19, 1966, Ser. No. 602,850 Clai 9 pis priority application Great Britain, Dec. 23, 1965, 54,510165 U.S. Cf. 176-68 6 Claims Int. Cl. G21c 3116 ABSTRACT OF THE DISCLOSURE For a fast nuclear reactor, a fuel element of the kind having ceramic fuel in metallic sheathing, wherein the fuel is divided into a central zone and an outer zone or iiiterlayer interposed contiguously between the central zone and the sheath, and fissile fuel is predominantly in th ' e central zone -with fertile fuel predominantly in the interlayer which has thermal insulating properties whereby thp operating temperature of the fuel can be increased so as to lower its compressive creep resistance to enable irradiation-induced swelling of the fuel to be absorbed in voidage distributed in the fuel by virtue of its aggregated density being arranged to be less than (for example 85% of) maximum theoretical density. Where plutonium fissi le atoms are employed, they are included in the central zone, while the interlayer is of natural or depleted -uranium. The present invention relates to fuel elements for fast nuclear reactors having fissile and fertile fuel, which is preferably ceramic, in a closely fitting metal sheath. Current research on ceramic fuels is showing that fuel swelling can be a prime cause of fuel element failure and thereby produce a Iiinit on attainable burn-up of heavy atoms. There are strong economic incentives to achieve as high a fraction of heavy atom burn-up as possible in fast reactors. A sigpificant feature of fuel swelling which is caused by the solid and gaseous fission products is that it can occur in such a way as to extend the perimeter of the fuel, thereby stretching the closely fitting sheath while at the same time leaving an unoccupied voidage in the central region of the fuel element. The basic reason for this is that the oiiter rim of fuel which swells continuously under irradiation behaves like a continuous expanding circular arch. The idea of a continuous arch is only tenable if the fuel is strong enough in its outer regions to stretch and cause failure of the sheath. At typical surface fuel temperatures for fast reactors, this is generally the case, since, although ceramic fuel does tend to become plastic at high temperatures, it is not likely to be adequately plastic below 1,000' C. l,t is an object of the invention to overcome this difficulty by arranging for fuel swelling to be associated mainly with hotter and therefore more, plastic fuel, so that as,swelling proceeds the fuel will flow or deform into the available voidage. At the same time, it is highly desirable that this should be done wilthout either loss of avera.-e fuel density in the reactor care as a whole, since this has an important effect on the economics of the system, or with increase of sheathing temperature, since sheath strength is liable to significant reductions with temperature increase. These three objectives can be simultaneously achieved, according to the present invention, by arranging that the fissite fuel is predomiriantly in a central zone and the fertile fuel predominantly in an outer zone interposed contiguously between the central zone and the sheath. With this arrangement fuel burnup will take place predominantly in the central zone where the Patented Jan. 14, 1969 2 fuel is hot and plastic. The outer zone will be subject to a certain amount of bum-up ;but in the event that fuel swelling problems do prove to be a limiting factor on attainable burnup, a two-zone form of fuel in accordance with the invention can be expected to, -ive a significant extension of that limit. Where plutonium is used to provide the enrichment required for a fast nuclear reactor the central zone may comprise a ceramic compound of plutonium and the outer 10 zone a ceramic compound of natural or depleted uranium. Preferably theso compounds are of the same type, for example, dioxides, and are in direct contact at the interface . of the zones. This direct contact avoids any need for the'provision of a peripheral layer on either of the zones 15 to :ict as an interlayer between them. Although fuels showing greatest reduction of creep sftrength with temperature will be preferred, the invention is applicable gener@lly to any ceramic fuel, such as oxide, carbide, silicide and nitride. 20 There are various ways in which the arrangement in accordance with the invention can be achieved in practice. These will be further explained with the aid of the accompanying drawings in which: FIG. I is a longitudinal section of a fuel assembly for 25 a liquid metal cooled fast reactor, the fuel in this assembly being contained in fuel pins, and FIGS. 2 and 3 are cross sections to an enlarged scale throiigh two forms of fuel pin embodyin- the invention in a manner suitable for use in the fuel assembly as shown 30 in FIG. 1. In FIG. I an outer hexagonal casing 10 of the assembly has a bottom fitting @ll comprising spaced cylindrical bearing surfaces 12 and 13 for fitting into a socket of a reactor core support structure so that the assembly is 35 supported in cantilever fashion by this structure. Between the bearing surfaces there is a stainless steel knitrnesh filter 14 which enables coolant (assumed in these examples to be sodium) supplied to an inlet plenlim incorporated in the core support structure to enter into the 40 bottom of the casing 10 and to be forced upwards therein. When positioned in the core eachassembly is separated from its neighbours only by narrow gaps which are predetermined by the pitch of the sockets in the core support structure and by corner abutments or pads such as 15 and 4,5 16 projecting from the casing. The fuel pins indicated 17 are clustered within the casing 10 in parallel array on a triangular lattice, the lattice pitch being large enough to ensure that the pins @do not come into contact with one another; they float 50 captively between top and bottom support plates 18 and 19 and are located transversely at intervals along the length of the cluster by grid structures (not shown). The coolant forced upwards in the casing therefore flows 55 longitudinally over the pins for removal of the heat generated thereby. The fuol pin appearing in section in FIG. I illustrates how a void length 20 amounting to about half the length of each pin to act as a reservoir for gases released by the 60 fuel in service is atrangetd at the lower end. This void length is provided since the pins are assumed to be of the sealed', type. Above the void length there are three sections represented diagrammatically, namely a lower bredder section 22, a fuel section 23 and an uppet breeder section 65 24. It is with the fuel section that the invention is prirnarily concerned and therefore the content of the breeder sections can be arranged in any way which is appropriate in combination with the fuel section arrangements now to be described with reference to FUGS. 2 and 3. 70 In FIG. 2 the fuel contained within thin-walled cylindrical sheathing 25 is divided into two zones having different concentrations of fissile atoms, the zone 26 of
[2]3)421,979 3 higher fissile atom concentration being at least for the most part inside the outer zone 27. In the extreme, the inner zone 26 may have concentrated in it practically all of the fissile content of the fuel while the outer zone 27 is principally of fertfle material. Thus, where plutonium atoms are employed for enrichment, the inner zone may be solely ceramic plutonium while the outer zone is of ceramie uranium which is prefera!bly depleted in fissile isotope. Although the fissile atoms are advantageously segregated to the utmost into the inner zone, it is not necessarily best that all the fertile atoms are segregated into the outer zone. If the segregation were to be so complete, it is likely that the temperatures towards the centre would exceed the melting point of the inner zone, zspecially ifthis zone is constituted by an oxide. Oxides characteristically have low thermal conductivities and therefore in FIG. 2, which is based on the use of oxide fuel, the plutonium atoms in the inner zone 26 are diluted with fertile uranium atoms. In greater detail, the inner zone is of (UPU)02, possibly to a slightly nonstoichiometric composition, with the uranium at the natural concentration of the 235 isotope and with the plutonium enrichment @at a level of about 27% for example. The outer zone 27 is a ceramic compound of the same type so that the two zones can The in direct contact without any need for an extra layer between them. The outer zone in FIG. 2 is therefore natural or depleted U02- In conjunction with the two-zone arrangement thus far described, it is essential to provide distributed voidage. Based on the provision of between 1 and 2% voidage per designed percent of maximum burn-up of heavy atoms, an aggregate voidage amounting to at least 15% of the fuel volume is considered appropriate. Primarily such a figure of voidage is related to the inner zone where the majotity of fission occurs. The same criterion does not apply to the same extent to the outer zone. Therefore the density of the,outer zone may be different from fliat of the inner zone, -for example higher, but in the present example it will be assumed that the densities are approximately the same. Stacked pellets of which the inner zone is composed are presintered to the requisite density, for example, about 80% of the maximum theoretical density. If, in the altemative, the pellets were to'be made in an annular shape, the central hole would contribute to the requisite voidage and less would be needed as porosity in the fuelThe outer zone may be introduced as powder in which case the packing density is controlled. Locating ribs, such as 28, are formed on the pellets of the inner zone for centering in the sheathing 25. The thickness of the outer zone in FIG. 2 is intended, on the basis of the heat @generation in the inner and,outer zones being in the ratio of 22.6: 1, to make the inner zone surface (excluding the ribs 28) about 400' C. hotter than the outer surface of the outer zone. With the fuel pin in service at the design heat output rating this outer surface is likely to have a temperature between 700 and 800' C. in the region of maximum 'bum-up,of the fuel, i.e. approximately the mid4ength of the fuelled section of the pin, -and therefore the inner zone surface temperature can be expected to exceed 1,000' C. By virtue of this surface temperature, all of the enriched fuel, at least in the region of maximum burn-up of the fuel pin, :becomes sifbject to atemperature greater than 1,000' C. and consequently there is created througliout this fuel such reduced resistance to compressive creep that irradiation-induced swelling can -be absorbed in the local voidage without exerting undue pressure on the outer zone, that is to say, pressure which will overstrain the metal sheathing. An indication of the extent to which the compressive creep strength of ceramic fuels is reduced with increasing temperature can be gained from the paper by Armstrong et al. at pp. 133 to 141 of Journal of Nuclear Materials, 7 No. 2 (1962). Alt,hough the ribs 28 will be cooler and hence stronger than the rest of the inner zone, they are no longer part 4 of a continuous arch and by being well separated may not exert any greater pressures on the outer zone or sheathing; even if they prove to 'be virtuahy undeformable it is conceivable that they will tend to eiribed themselves into the main'body of the inner zone. The ribs therefore illustrate that the interlayer Tepresented by the outer zone can serve the intended function merely @by rendering discontinuous such portions of the enriched fuel as are in direct heat transfer relationship with the metal sheathing. 10 The surface temperatures quoted above are referred to the maximum burnup region of the fuel because m @,an elongated fuel element it is generally not feasible or ev@n necessary to -aim for such high surface temperatures to-wards the ends of the element. Typically there is a down15 war temperature -gra lent towar s t en s w i rer sults from a lowering of neutron density and fission rate. The lower fission rate implies less swelling tendency and hence less need for measures to avoid swelling at the external surface. 20 Some fission will occur in the outer zone and is likely to increase as burn-up proceeds since more fertile material is present in the outer zone than the inner zone and therefore mor-e new fissile atoms per unit volume are b@red into the outer zone. For example, an initial fission rate distri25 bution of 5:1 could reduce to 341 at an average !burn-up of 10%. Nevertheless, it can 'be expected that an extension of burn-up limits would be obtainable and there is the advantage that maximum use is made of fertile material to 30 assist in breeding. FIG. 3 is a modification of FIG ' 2 using ceramic fuel of higher thermal conductivity, such as monoca@bide. The outer zone 29 is continuous and of uniform thickness and is of natural or depleted UC; since it is thicker than the 35 equivalent outer zone in FIG. 2 it is conveniently made in the form of hollow annular pellets into which the enriched fuel, i.e. (UPu)C, is packed as p6wder to form the inner zone 30. As before, the plutonium enrichment lies exclusively in the inner zone. 40 In general the manufacture will depend orl the nature of the materials, the required thicknesses and the densities. Apart from processes involving preformin.- one component and filling the other as powder, others which are open to consideration according to the circumstances are 45 flame spraying and coextrusion. For densifying packed powder interlayers, or simply for ensuring tight engagement with the sheathing, use could be made of a rotary swaging or explosive forming operation performed on the sheathing after filling. 50 While the above examples deal specifically in many respects with particular circumstances, it will be appreciated that many variations of the invention are possible within the scope as defined by the appended claims and without departing from the spirit and general purpose of 55 the invention as herein described. With regard to such variations, a possibility which may be worthy of mention is that of adding to the interlayer a thermal insulating material which idoes not contain fissile or fertile atoms. Such material would be a diluent for atoms of these kinds already present in the interlayer. For instance, if beryllia 60 is required for countering reactivity gain on loss of coolant !n the reactor, such beryllia may be mixed in with the interlayer. It also follows that an interlayer in accordance with this invention could be used in place of the one withr,5 out fissile or fertile atoms which is employed in an application of even date by K. M. Swanson at the external cooled surface of a type of fuel element having intemal as well as external cooled surfaces. What I
